Page 62 - Valve Selection Handbook

Manual Valves 49

Flange standards may offer a variety of flange face designs and also rec-
ommend the appropriate flange face finish. As a rule, a serrated flange face
finish gives good results for soft gaskets. Metallic gaskets require a finer
flange finish for best results. Chapter 2 discusses the design of gaskets.

Welding end connections. These are suitable for all pressures and tem-
peratures, and are considerably more reliable at elevated temperatures
and other severe applications than flanged connections. However,
removal and re-erection of welding end valves is more difficult. The use
of welding end valves is therefore normally restricted to applications in
which the valve is expected to operate reliably for long periods, or appli-
cations which are critical or which involve high temperatures.
Welding end valves up to DN 50 (NFS 2) are usually provided with
welding sockets, which receive plain end pipes. Because socket weld
joints form a crevice between socket and pipe, there is the possibility of
crevice corrosion with some fluids. Also, pipe vibrations can fatigue the
joint. Therefore, codes may restrict the use of welding sockets.

Standards Pertaining to Valve Ends

Appendix C provides a list of the most important U.S. and British
standards pertaining to valve ends.
Valve Ratings

The rating of valves defines the pressure-temperature relationship
within which the valve may be operated. The responsibility for determin-
ing valve ratings has been left over the years largely to the individual
manufacturer. The frequent U.S. practice of stating the pressure rating of
general purpose valves in terms of WOG (water, oil, gas) and WSP (wet
steam pressure) is a carryover from the days when water, oil, gas, and
wet steam were the substances generally carried in piping systems. The
WOG rating refers to the room-temperature rating, while the WSP rating
is usually the high temperature rating. When both a high and a low tem-
perature rating is given, it is generally understood that a straight-line
pressure-temperature relationship exists between the two points.
Some U.S. and British standards on flanged valves set ratings that equal
the standard flange rating. Both groups of standards specify also the per-
missible construction material for the pressure-containing valve parts. The